To install, you need to have the WordPress i18n library on your computer. Check it out using SVN:
sudo svn co http://i18n.svn.wordpress.org/tools/trunk/ /usr/lib/wpi18n
You don't have to put the library in /usr/lib/wpi18n, but if you don't put it there, make sure to set the $WP_I18N_LIB environment variable in your .bashrc or .bash_profile file (with no trailing slash):
export WP_I18N_LIB="/path/to/i18n/lib"